This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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adj. Impossible to placate or appease: implacable foes; implacable suspicion.

adj. Not able to be placated or appeased.

adj. Adamant; immovable.

adj. Not placable; not to be appeased; incapable of being pacified; inexorable.

adj. Incapable of being relieved or assuaged; inextinguishable.

Not placable; not to be appeased; not to be pacified or reconciled; inexorable: as, an implacable prince; implacable malice.

Not to be relieved or assuaged.

Synonyms Relentless, etc. (see inexorable), unappeasable, unforgiving, vindictive, pitiless, rancorous.

adj. incapable of being placated

"So implacable is his revenge that he sends a party of soldiers into Egypt, some hundreds of miles, and they bring him back by force of arms."

grim    unforgiving    unmitigable    relentless    stern    inexorable    unappeasable    unrelenting   

merciful   

unforgiving    mortal   

relentless    irreconcilable    unyielding    inexorable    unappeasable    unrelenting    inextinguishable    impropitiable    immitigable    deadly   

relentless    inexorable    vindictive    ruthless    remorseless    inflexible    fierce    imperious    sullen    stubborn